In the PDF, the car is listed as "#4999 Air Jet Car", but no further description. Alex, Steve, JonnK and I have searched for any proto pics, but we've been unable to find any. We did find several pictures of a 3 hopper car manufactured by Haliburton, but none of this 4 hopper car.

George Irwin wrote: I don't have a lot of SP information-- I depend on some of my readers to augment what's in the UMTRR Research Accumulation

I am going to guess that [this car] is a variation on a cement carrier. In the east, railroads including the NYC and the Lehigh Valley converted gondolas to carry pressurized cement containers. MTL did a pair of NYC gondolas with a depiction of the cement container load (105 00 72x, March 2010).

SP wasn't well known for washing its locomotives (!), but they were pretty innovative in other areas. (The SP in "Sprint" is for Southern Pacific, for example, as they were a pioneer in microwave communication which led to that alternative long distance carrier.) It wouldn't surprise me if they experimented with a car like this before electing to go with a more traditional covered hopper like the Airslide.

Side note: If any of you are unaware of
UMTRR
, I would highly suggest that you subscribe to Mr Irwin's monthly musings.. they're a very interesting critique of MTL releases, as well as a bit of history behind each car. VERY entertaining reading, and his depth of knowledge amazes me.

Update: Unfortunately, Shapeways has rejected the model because the railings are too thin. I'll have to go back and thicken them up a bit.

I think that trainboard modeler wasn't one of those great modeler he seem to blame what he can't fix! the cylinder car looks great ! I think he just didn't drill the holes out right. I had one of mine do that the truck didn't ride right! but it does now! You were a good teacher!